The present invention relates to transducers of the cantilevered beam type wherein a force to be measured is applied perpendicular to the end of the beam through a joint and, more particularly, to a novel transducer and the improved joint used therein comprising, a self-aligning first ball bearing having an axially aligned center bore and a cylindrical outer surface; a cylindrical inner member carried by the end of the beam adapted to receive the center bore as a snug fit thereon; a cylindrical outer member having cylindrical inner and outer surfaces, the cylindrical inner surface being of a diameter to be a sliding fit about the cylindrical outer surface of the first ball bearing, the cylindrical inner surface also having parallel, spaced, inward-facing ridges thereon of a spacing greater than the width of the first ball bearing, the cylindrical outer member further including means for receiving a force to be measured; bias means disposed between the inward-facing ridges and the first ball bearing for resiliently urging the first ball bearing towards a neutral position; a second ball bearing having an axially aligned center bore of a radius larger than the radius of the cylindrical outer surface of the cylindrical outer member by an amount equal to the maximum deflection to be allowed, the secnd ball bearing being disposed concentrically about the cylindrical outer member in alignment with the first ball bearing; and, means for non-movably supporting the second ball bearing with respect to the support of the cantilevered beam.
Tension transducers are well known in the art and are manufactured and sold commercially by a number of companies including the applicant's company, Dover Flexo Electronics, Inc. A web tension transducer is used to measure tension in any moving web. The output signal produced can be displayed for manual control of the tension or can be used for automated control. They are typically used on printing presses, coaters, laminators, textile machines, slitter-rewinders, sheeters, paper making machines, and any other machine which in some way processes a continuous web of paper, plastic film, metal, textile, non-woven textile, felt, rubber, or any combination of the foregoing. Variations of web tension can cause web breakage, slack web, wrinkling, curling, stretching, and in general, poor quality product and excessive waste. These problems can be eliminated by proper control of web tension. The basis of proper control is tension measurement. The basis of measurement is a transducer which will convert a force (web tension) into an electrical signal which can be used for control purposes. The transducer must be accurate regardless of web speed and tension or it will not allow the correction of the aforementioned problems.
A typical application for such transducers is shown in simplified form in FIG. 1 wherein a moving web 10 is directed over rollers 12 and 14. The non-rotating shaft 16 of roller 14 is supported on the ends by tension transducers 18. The tension in the web 10 is sensed by the transducers 18 which, in turn, send electrical signals to the control circuit generally indicated as 20. The control circuit 20 then controls a brake 22 on the supply roll 24 to adjust the tension in the web 10 to a pre-established/adjustable level.
A prior art tension transducer and the joint contained therein for connecting to the shaft 16 is shown in simplified form in FIG. 2. The transducer 18 comprises a beam 26 extending horizontally outward from a rigid support at 28. Strain gauges 30, of a type well known in the art, are attached to the beam 26 and provide the above-described electrical signals as a function of any bending of the beam 26. The outer end of the beam 26 is provided with a joint, generally indicated as 32, by which the beam 26 is connected to the shaft 16. The joint comprises an inner member 34 and an outer member 36. Since the shaft 16 is non-rotating, the joint 32 is an adaption of the U-joint construction employed with rotating shafts such as the drive shaft of an automobile. The outer member 36 has a socket 38 for receiving the shaft 16. A yoke 40 straddles the inner member 34 and is free to pivot in the direction of the arrows 42 around balls 44, which are disposed between the yoke 40 and the inner member 34, where they are located by sockets (not shown) in the inner member 34 and by slots (not shown) in the yoke 40. The slots in the yoke 40 allow limited longitudinal movement in the direction of the arrow 46. The pivoting action of the joint 32 allows for minor misalignment while the longitudinal sliding action of the balls 44 in the slot allows for differences in shaft length as well as expansion and contraction due to temperature differences. The roller 14 is mounted on the shaft 16 by means of ball bearings such as that indicated as 48. Beam deflection is measured in a manner well known in the art by placing a pair of strain gauges 30 disposed as shown on opposite sides of the beam 26. The electrical signals from the two gauges 30 are then arithmetically processed to eliminate the portion resulting from the twisting of the beam 26.
The deflection of an end-loaded cantilever beam (such as beam 26 described above) is very linear with applied load if the deflection is small. The amount of deflection at the end of the beam is given by the formula Y=PL.sup.3 /3EI; where, Y is the deflection, P is the load, L is the beam length, E is the modulus of elasticity of the beam material, and I is the moment of inertia of the beam. This formula is valid when the load is applied perpendicularly to the beam, which is not true if the deflection is large. Also, the formula only applies to pure cantilever bending. No other forces or moments can be applied or the linearity and accuracy of the deflection/load relationship will be affected. All deflection must be caused by the applied load alone. The purpose of the joint 32 (or its equivalent) is to ensure this is true to the highest possible degree.
The joint is necessary because web tension transducers are used in pairs. The transducers are mounted firmly on the inside surfaces of opposed machine frame members and a strong shaft is clamped into them. The web passes over the shaft (or, more typically, a tube rotating on bearings on the shaft as described above) and produces a force perpendicular to the shaft proportional to web tension. The machine frame members are immovable. If the shaft were clamped directly to the beam, instead of through a joint, a bending moment would be applied to the end of the beam and the deflection curve would be non-linear. Also, the shaft is not perfectly rigid and will bend as load is applied. The bending shortens the distance between the ends slightly producing an effective reduction in length as well as a change in angle at the ends. The effects of the bending moment, distance change, and angle change must be eliminated if the deflection of the beam is to be linear with applied load. The joint must be designed to do this.
As will be readily recognized from the above discussion relative to the prior art tension transducer of FIG. 1 in general and the joint thereof in particular, there are certain drawbacks associated therewith which would be desirable to correct to realize an improved tension transducer and an improved joint for use in that tension transducer, as well as in any similar application. For one thing, the transducer of FIG. 1 cannot be used with a rotating shaft. For another, a separate bearing must be used to support the weight of the shaft. Also, there is no adequate provision for protection for the transducer in the event that there is too much deflection for any reason or in the event that a momentary bearing seizure, or the like, imparts a large rotational force on the shaft and into the transducer. In these latter instance, of course, the prior art transducer can be seriously damaged. Yet another consideration of the prior art transducer and its joint is the tolerance/friction tradeoff; that is, if the fit of the pin and slot is tight enough to prevent error-producing slop, there may be considerable friction, which could, itself, introduce an error into the tension measurement.
Wherefore, it is an object of the present invention to provide a tension transducer of the cantilever beam type which can be used with either non-rotating or rotating shafts.
It is a further object of the present invention to provide a tension transducer of the cantilever beam type which incorporates safety provisions against overload, overbending, and rotational torque.
It is yet another object of the present invention to provide a tension transducer of the cantilever beam type which provides a tight tolerance joint with low friction.
Other objects and features of the present invention will become apparent from the discussion and accompanying drawings contained hereinafter.